Because of its low cost and compact size, the solid state (GaAs) power amplifier (SSPA) is a commonly employed component of the uplink transmitter of a VSAT (very small aperture terminal) Ku band earth station. The (gain) transfer (power output vs. power input) characteristic of a typical SSPA has a positive sloped linear region which gradually decreases over a knee portion to a slightly negative-sloped saturation region. When driven at the 1.0 dB gain compression point, namely at that point on the knee of the transfer characteristic which is 1 dB below an extension of its linear, positive slope region, the frequency response of the SSPA has a substantially smooth profile over its intended operational range.
for currently available SSPAs the output power at the 1 dB gain compression point is on the order of two watts, which may be considerably greater than the power required by the link (e.g. one watt). Consequently, it is necessary to reduce the output power transmitted by the uplink terminal. Idealy, this power reduction would be accomplished by simply reducing the input power to the SSPA, as its output linearly tracks its input over the linear portion of its transfer characteristic below the 1 dB compression point. Unfortunately, the frequency response of the SSPA does not follow suit. Instead, the frequency response drastically departs from the smooth flat profile at 1 dB compression and ripples significantly across the transmit passband.
In copending U.S. patent application entitled "Power Control System for VSAT Earth Station", by William E. McGann, filed Sept. 15, 1986, U.S. Ser. No. 907,009, assigned to the Assignee of the present application now abandoned, there is described a signal processing scheme for compensating for this nonlinear frequency response and additional performance variables (e.g. gain vs. temperature) which essentially comprises a preemphasis mechanism inserted upstream of the SSPA. Now, while the nonlinearity compensation scheme described in that application is certainly a useful technique for solving the nonlinearity problem, it would be preferred, if possible, to operate the SSPA in such a manner to avoid the problem altogether.